The rectoanal region of the GI tract regulates the final storage, transport and evacuation of the GI contents. These functions require substantial differences in the contractile behavior of progressively more distal segments of this region. This study investigates the mechanisms underlying spontaneous motility patterns and the actions of excitatory nerves in rectoanal circular muscle of the dog and mouse gastrointestinal tract. The overall hypothesis for these studies is that fundamental changes occur in the nature of pacemaker activity, neuronal innervation and the actions of norepinephrine (NE) and acetylcholine (ACh) which permit adjacent portions of the rectoanal region to perform complimentary but distinct functions. Membrane potential and contractile activity will be measured as well as receptor density using radioligand binding techniques. These data will be compared to immunohistochemical and immunocytochemical measurements to identify putative pacemaker cells (i.e., interstitial cells of Cajal or ICC) and their relationship to intrinsic and extrinsic nerves. An 8 cm region of the canine model and a 1 cm region of the mouse model encompassing both the rectum and internal anal sphincter (IAS) will be used. The specific aims of this proposal are: 1) To compare the spatial distribution of pacemaker potentials to ICC, 2) To characterize the changes which occur in functional motor innervation with distance in the rectoanal region, 3) To determine the anatomical distribution of nerves and their relationship to the distribution of ICC, 4) To evaluate postjunctional adrenergic and muscarinic responses in the rectoanal region using functional measurements and radioligand binding techniques, 5) To evaluate pacemaking using knockout mice and organ culture techniques. Millions of Americans suffer from diseases of the rectoanal region which lead to either constipation or fecal incontinence. The experiments outlined in this proposal will provide important new information regarding the anatomical and physiological characteristics which underlie rectoanal motility and its nervous control. In so doing we may aid future studies directed toward understanding how these functions become altered in disease states.